Machine for the continuous extru



Sept. 12, 1939. P bu s 2,172,651

MACHINE FOR THE CONTINUOUS EXTRUSION OF METAL Filed May 3, 19-37 4Sheets-Sheet 1 Fig. 1. 3

p 2, 1939. P. DUNSHEATH 2,172,651

MACHINE FOR THE CONTINUOUS EXTRUSION OF METAL I Filed May 3, 1937 4Sheets-Sheet 2 Fig. I!

15a 15d [5c 15b 15a 10 l/ 5 I y Sept. 12, 1939. P. DYUNSHEATH 2,172.651

MACHINE FOR THE CONTINUOUS EXTRI ISION OF METAL Filed May 3, 19-37 4Sheets-Sheet 3 p 2, 1939; I P. DUNSHEATH ,1

MACHINE FOR THE CONTINUOUS EXTRUSION 0F METAL Filed May 3, 19-37 4Sheets-Sheet 4 1 Fig. '5.

MYM

Patented Sept. 12, 1939 UNITED STATES PATENT OFFICE MACHINE FOR THECONTINUOUS EXTRU- SION F METAL Application May 3, 1937, Serial No.140,545 In Great Britain May 11, 1936 30 Claims.

This invention relates to the extrusion of metal by a method employing arotating screw-threaded impelling device by which the metal is drivenforward into an extrusion chamber and from this through an extrusionorifice formed by a die or between an inner and an outer die. Theimpelling device may comprise two longitudinally extending concentricmembers of circular crosssection which have on their adjacent faces promjections which are adapted to engage metal placed in the space betweenthe members. One or both of these sets of projections are screw threads.They work over each other with small clearance. The metal is fed intoone end of the. space between the two members in the molten state and isthen cooled so that it acquires sumcient rigidity to co-operate with theprojections. One or both of the two members is or are driven incontinuous rotation. The relative rotary- 20 movement of the twomembers, combined with continuous feeding of molten metal, causes thesolidified metal to be driven forward into and through the extrusionchamber which is located at the delivery end of the two members. An ex-25 ample of apparatus of this kind is described in the specification ofPatent No. 2,051,688.

The nature of the metal which can be extruded in this apparatus islimited to some extent by the form of the apparatus since the metal must30 have a reasonable degree of plasticity at a temperature at which theworking parts have the necessary strength and freedom of movement. Theapparatus has hitherto been worked with lead and lead alloys.

It has been found that the process of working as hitherto carried out insuch machines frequently results in the segregation of impurities orother constituents of the metal (which may be and generally are onlypresent in very small o quantities) to form layers within the structureof the extrudedbody. Such segregation while apparently not producingdeterioration of the mechanical properties of the body is to be avoidedif possible.

The causes and procedure of this-segregation are not clearly understood,but the result appears to be favoured, if not, actually caused, by theregularity of the movement of the metal within the machine, whichmovement is mainly a 50 sliding of the pasty or plastic metal under highpressure over the rigid and symmetrical metal surfaces of the impellingmembers and the extrusion chamber. The segregated layers generally lieparallel with the surfaces over which 55 the metal has moved or areformed and placed so as to indicate a definite relationship with thesesurfaces.

It has been recognized that segregation of impurities may occur in metalpresses of the ram type and that such segregation and its attendant 5disadvantages can be avoided or at least minimized by modifying thesurfaces of the passages, through which the metal flows from thecontainer to the extrusions die, in such a manner as to produce adisturbance of the regularity of the pathtaken by the metal suflicientto prevent a segregation taking place or (if it has occurred) to breakit up before the metal comes near to the extrusion die where thestructure of the metal is finally fixed. As indicated in the precedingparagraph, it is believed that segregation in extrusion machines of therotating screw thread impeller type takes place within the impellingdevice itself before the metal reaches the extrusion chamber in whichthe known means for counteracting segregation can be applied. It is theprincipal object of this invention to provide for such a machine amodified form of screw thread impelling device in which the occurrenceof segregation is prevented or at least reduced to a harmless degree,without any reduction in out-put of the impelling device. In ourimproved form of construction of impelling device, the depths of thegrooves of the threads or other channels in one of the two membersconstituting the device increases progressively as the delivery end ofthe device is approached. This progressive increase in depth may extendfrom the rear end to the front end of the member or from someintermediate point. The progressive increase as I the delivery end ofthe device is approached in the depth of the grooves of the threads orother channels in one of the members constituting the impelling devicemay advantageously be accompanied by a progressive decrease in the depthof the grooves of the threads or other channels in the other of the twomembers. The progressive increase in depth of the grooves of the threadsor other channels in one of the members causes a folding up of the metalin the channels of that 5 member and a gradual transfer of metal fromthe channels of the other member to those of the first member andconsequently a mixing of the metal passing forward along the channelsbut does not reduce the output 'of the impelling de- 5 vice. On thecontrary, such an increase in depth of the channels in the stationarymember has been found to give an unexpected increase in output. Furthermodifications to the impelling device may be made in order to increasethe mixing effect without appreciable reduction in output and these willbe described with reference to the accompanying drawings wherein,

Figures 1 and 1 together show a longitudinal section of parts of anextrusion machine embodying the invention,

Figure 1a is a fragmental longitudinal section on an enlarged scale ofthe front end of the impelling device,

Figure 2 is a cross-section on an enlarged scale, and taken on the linelI-Il in Figure 1 of the outer of the two concentric membersconstituting the impelling device, V

Figure 3 is a development of the forward end of the inner concentricmember,

Figure 4 is an end view on an enlarged scale of the forward end of theinner concentric member,

Figure 5 is a fragmental longitudinal section showing a modified form offront end support for the inner concentric member,

' Figure 6 is a fragmental cross-section on the line VI-V'I in Figure 5,and

Figure 7 is a cross-section on the line VII-VII in Figure 6.

It must be understood that the machine shown is, as -regards theconstruction and arrangement of its essential parts, such as, forinstance, thegeneral form of the impelling device, the number and pitchand shape of the threads or other projections on'the impelling membersand the method of drive, merely one example of machine to which theinvention has been applied. The way in which it may be applied to othermachines of the screw thread impeller type will become clear afterreading the description of the way in which the invention has beenapplied to this machine. It is also to be understood that Figure l, asregards the general construction of the machine, is to some extentdiagrammatic. It shows the relative disposition of the important partsof the machine but is not intended to indicate the details of theirconstruction, for the'present invention is concerned only'with theformation of the parts of the machine with which the plastic metal is incontact.

The construction and operation of the machine illustrated will firstbedescribed and the various modifications that have been made to the1m:-

. pelling device will be described subsequently. Re-,

ferring first of all to Figure 1, it will be observed that the rotarypart of the screw thread impelling device is constituted by the outertubular member I which is supported in bearings in the frame 2 anddriven by the wheel 3. At the inlet ported at its front end by membersI2 extendins between it and the wall of the extrusion chamher 9. At itsfront end the member Ill carries an inner die or pointf I! whichco-operates with an outer die H to form an annular extrusion orifice.The lower parts of the frame and covers and other parts below the outerconcentric member have been omitted as their inclusion is unnecessaryfor an understanding of the present invention. On similar grounds themeans for adjusting the outer die, and all heating and cooling deviceshave been omitted.

In the operation of the machine, molten metal is continuously fed intothe chamber I and flows into the space between the two members I and [0where it is cooled sufliciently to become plastic and to be engaged bythe threads l5 and ll, of

the stationary member II and the reaction of these threads assists toforce it forward. As a result of these actions, the metal is deliveredinto the extrusion chamber 9' and forced out between the dies I3 and M.The inner concentric member I0 and the die I! are tubular to permit ofthe passage of a core, such as an electric cable, when the machine is tobe employed for applying a metal sheath to such a body.

The description will now be given of the various modifications that havebeen made to the impelling device to prevent segregation taking placetherein. In accordance with the invention, the depth of the grooves ofthe threads I6 in the member Ill increases progressively, from a point,indicated by the line A--A, intermediate the length of the threadedportion, as the delivery end is approached. This increase in depth ofthe grooves in the threads of the member III is accompanied by aprogressive decrease in depth of the grooves of the threads in therotating member I. It will be seen that this arrangement results in agradual transposition of the plastic metal in an inward direction. Thismovement superposed on those which normally occur in the screw threadimpelling action causes a mixing of the metal. This modification m thedepth of the grooves on the channels does not involve an increase in theclearance between the tops of the threads of the two members and doesnot, therefore, reduce the effectiveness of the impelling device. Theprogressive increase in depth of the threads of the stationary part ofthe impelling'devices, although accompanied by a diminution in depth ofthe threads of the rotating part, may still provide a path for theplastic metal that increases progressively in cross-sectional area asthe front end is approached. This increase appears to favor theimpelling action.

Instead of, but preferably in addition to, imparting a progressivethread depth variation, continuous in the same sense, to one of theimpelling members, the depth of the grooves of the threads l5 or I orother channels in one or both of the members may be made to varyperiodically along the length of the member. As shown in Figure 1 thedepth of the groove of the thread in each start of the four start threadin the driver member i increases from a minimum to a maximum and thendecreases to a minimum again once during each length of lead of thethread. The variation is thus a periodic wave but the w'aves in adjacentthreads are not in phase with one another. This periodic variation indepth may be obtained by cuttingeach start of the thread whilst rotatingthe driver about an axis displacedfromtheaxislloftheboreofthe driver, asshown in Figure 2, the groove lIa being cut whilst the driver is rotatedon axis "a, the groove I5b whilst the driver is on axis "b and so on.Alternatively a similar effect may be obtained by rotating the driverabout its axis and cutting the grooves to form the thread whilst thetool is reciprocated towards and away from the axis of rotation so as tovary the depth of the groove periodically. This variation is clearlyshown in the rear part of the driver member I but is not apparent at thefront end because of the gradual diminution in depth previouslydescribed. This periodic variation in depth may be applied as shown toall the channels in the driver member or only to certain of thechannels. This periodic variation in the depth of the grooves alongwhich the stream of metal passes causes the metal to be forced at theshallow places to move out of the one member into a channel in the otherconcentric member, so that this additional movement is superposed onthose produced by the continuous thread depth variations previouslydescribed. Thus a more thorough mixing of the metal results.

In applying a modification of the channels as above described it appearsimportant that the modification should be made in those channels wherecooling takes place, since it appears probable that segregation is mostlikely to occur as the cooling proceeds.

Alternatively or in addition a. mixing of the plastic metal may beeffected by providing a local increase in the radial clearance betweenthe projections on the two members. By a local increase in clearance ismeant a clearance formed by providing a transverse groove or shallowdepression in the wall separating the adjacent channels in one or otherof the impelling members, or by equivalent means. ance is not to beconfused with a gradual progressive increase in clearance as the frontend is approached produced by a gradual dying away of the projections asdescribed in the specification of prior Patent No. Re. 18,710. Aprogressive increase in clearance of this kind, while it produces adisturbance in the flow of the metal, so seriously curtails the outputof the'machine as to be undesirable in present day practice. As shown inthe drawings, the walls separating adjacent channels in the innermember. III are mutilated at intervals along the length of the member bycutting grooves I8 therein. The grooves may be either circumferential orlongitudinal grooves but the former (shown in Figure I) have been foundto be more efficient than the latter. These grooves are generallynarrow, as

shown. They may extend to the bottom of the channels or may extend for apart only of the depth of the separating wall. By this means the metaltraveling along one channel is at one or more points allowed to slipthrough into the succeeding channel and mix with the metal there. Thismovement naturally produces a disturbance of the regularity of flow aswell as a mixing between the two streams of metal. It is obvious thatthe mutilation of the wall by the narrow grooves must affect acomparatively small part of the walls of the channels, so that thegeneral course of the impelling action is not broken up.

A little to the rear .of the delivery end of the impelling device it ispreferred to provide a more extensive local increase in clearance bycutting or otherwise forming atransverse groove 26 of considerable widthin each of the threads II or other ribs on the stationary part of theimpelling Such a local increase in clear device. Preferably this grooveis of tapered form, having a maximum depth at or near its leading edge21 and zero depth at its rear edge as shown in Figures 3 and 4. FromFigure 3 it wfll be seen that the width of each of the grooves isapproximately equal to half the length of one complete turn of thethread in which it is cut. Preferably also the grooves do not extend tothe bottom of the channels. This local increase in clearance allow aconsiderable amount of metal to be carto intermingle before leaving theimpelling device to enter the extrusion chamber. It has been found thatsuch grooves in the channel walls,

I especially the wide groove near the front end of the impelling device,are of considerable benefit in that they have the effect of equalisingthe pressure in adjacent channels, thus ensuring stability of operation,by which term is meant the ability of the machine to continue for aconsiderable period to produce a tube of uniform wall thickness, andincreasing the continuity of action, that is to say, it increases theperiod of time during which the machine will continue to extrude withouta falling on in output which, for reasons not fully understood,sometimes occurs in machines as hitherto constructed.

At the delivery end-of the impelling device a supporting means aregenerally required for the inner member. These are disposed at intervalsaround the periphery of the inner member and extend between this memberand the wall of the extrusion chamber. In the machine shown in Figure 1they are integral with the inner member l0 and project outwards as shownat l2 and are constituted by the end parts of the threads 1.6 or otherwalls between the channels in the inner member I, the pitch of thethreads l6 being increased in the region of the front end to permit ofthem being broadened without reducing the width of the channels, asshown clearly in Figures 3 and 4. In the former case they areconstituted by the end parts of the threads I6 or other walls betweenthe channels in the inner member ID. The pitch of the threads I6 isincreased in the region of the front end to permit of them beingbroadened without reducing the width of the channels, as shown clearlyin Figures 3 and 4.

The side walls of the end parts of the threads forming the supports l2have corrugated surfaces, the corrugations extending to the front face2| of the support and being inclined at an angle to the generaldirection of the stream. These may be formed by providing the walls withgrooves 22 as shown in Figures 1, ,4 5 and 6 or by providing ribs on thewalls. I'hese corrugated surfaces give to the path of the stream whichis in each other and thereby produce something in the nature of adiffusion of the meeting streams which assists in counteracting anytendency to segregation that may be caused at the front extremity of theimpelling device. In cases where the supports project inwards from thewall of the chamber as shown at in Figures 5, 6 and '7, similarmodifications may be made to the lateral walls thereof.

The form of the stream may also be diversified as it leaves the channelin the propelling member by forming a local recess running along in thebase of the channel. Such a recess will give extra depth to the channelover a narrow width and accordingly will disturb the transverse form ofthe stream.

It will be appreciated that although a machine has been described thathas an impelling device incorporating several modifications in additionto the modification which comprises imparting to the grooves of thethreads or other channels of the one member a progressive increase indepth as the delivery end is approached, the latter modification may beemployed by itself or in combinationwith any one (or more) of the othermodifications.

What I claim as my invention is:

1. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the adjacent surfaces of said members which co-operate toform a screw thread device for impelling plastic metal between saidmembers, the projections on one of said members being helical ribsseparated by a helically extending channel of which the depth variesperiodically, thereby producing a disturbance of the regularity of thepaths taken by the plastic metal without interference with the generallines of these paths.

2. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the adjacent surfaces of said members which co-operate toform a screw thread device for impelling plastic metal between saidmembers, the projections on one of said members being formed bycuttingat least two helical channels in the surface thereof, one of saidhelical channels having its axis displaced in one direction from thesaid common axis of rotation which is also the axis of the said surfaceand the other of said channels having its axis displaced in anotherdirection from the said common axis oLrotatipn thereby forming channelsof which the depth varies periodically alon the length' thereof.

3. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the adjacent surfaces of said members which co-operate toform a screw thread device for impelling plastic metal between saidmembers, the projections on one of said members being helical ribsseparated by a helically extending channel of which the depth increasesprogressively as the delivery end of the device is approached.

4. In a. machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the adjacentsurfaces of said members which co-operate toform a screw thread device for impelling plastic metal between saidmembers, the projections on one of said members comprising helical ribsseparated by a channel of which the depth increases progressively as thedelivery end of the device is approached and the projections on theother of said members comprising ribs separated by a channel of whichthe depth decreases progressively as the delivery end of the device isapproached.

5. In a machine for the extrusion of plastic metal, comprising twomembers relatively rotatable about a common axis and projections on theadjacent surfaces of said members which cooperate to form a device forimpelling plastic metal between said members, the provision of a localincrease in the clearance between the said co-operating projections,thereby producing a disturbance of the regularity of the paths taken bythe plastic metal without interference with the general lines of thesepaths.

6. In a machine for the extrusion of plastic metal, two membersrelatively rotatable about a common axis, projections on the adjacentsurfaces of said members which co-operate to form a device for impellingplastic metal between said members, the projections on one of saidmembers comprising a rib from which portions have been locally removed,thereby-producing a disturbance of the regularity of the paths taken bythe plastic metal without interference with the general lines of thesepaths.

7. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the adjacent surfaces of said members which co-operate toform a device for impelling plastic metal between said members, theprojections on one of said members comprising helical ribs in which atintervals along the length thereof are narrow circumferentiallyextending slots for producing a disturbance of the regularity of thepaths taken by the plastic metal without interference with the generallines of these paths.

8. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members, means for driving one of said members inrotation about its axis, means for anchoring the other of said members,projections on the adjacent surfaces of said members which co-operate toform a device for impelling plastic metal between said members, and apre-forming chamber in which the several streams of plastic metal arecaused to intermingle before leaving the impelling device, said chamberbeing located between the inlet and delivery ends of said impellingdevice andformed in the stationary member by providing a local increasein the clearance between the projections on the stationary member andthose on the rotating member.

9. In a machine for the extrusion of plastic metal, two relativelyrotatable and longitudinally extending, concentric members, projectionson the adjacent surfaces of said members which cooperate to form a.screw thread device for impelling plastic metal between said members, anextrusion chamber at the delivery end of said device, supports extendingbetween the inner of said concentric members and the wall of saidchamber, said supports having corrugated side walls, the corrugationsextending at an acute angle to the general direction of the stream ofplastic metal flowing therepast.

10. In a metal extrusion machine as specified in claim 9, thedisposition of the corrugations on the-one side wall of a support at anangle to those on the opposite side wall. of that support.

11. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the adjacent surfaces of said members which cooperate toform a screw thread device for impelling plastic metal between saidmembers, the projections on one of said members comprising ribsseparated by a channel of which the depth increases progressively, froma point between its ends, as the delivery end of the device isapproached.

12. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members, means for driving one of said members inrotation about its axis, means foranchoring the other of said membersagainst rotation, projections on the adjacent surfaces of said memberswhich co-operate to form a screw thread device for impelling plasticmetal between said members, the projections on said anchored membercomprising ribs separated by a channel of which the depth increasesprogressively from a point between its ends, as the delivery end of thedevice is approached.

13. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the adjacent surfaces of said members which co-operate toform a screw thread device for impelling plastic metal between saidmembers, the projections on one of said members comprising ribsseparated by a channel of which the depth increases progressively, froma point between its ends, as the delivery end of the device isapproached, and the projections on the other of said members comprisingribs separated by a channel of which the depth decreases progressively,from a point between its ends as the delivery end of the device isapproached.

14. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the adjacent surfaces of said members which co-operate toform a screw thread device for impelling plastic metal between saidmembers, the projections on one of said members comprising helical ribsseparated by .a channel of which the depth increases progressively, froma point between its ends, as the delivery end of the device isapproached, and the projections on the other of said members comprisingribs separated by a. channel of which the depth decreases progressively,from a point between its ends as the delivery end of the device isapproached.

15. In a machine for the extrusion of plastic metal, two concentricmembers relatively rotatable about a common axis, projections on theadjacent surfaces of said members which cooperate to form a screw threaddevice for impelling plastic metal between said members, the projectionson one of said members comprising ribs separated by a channel of whichthe depth increases progressively, from a point between its ends, as thedelivery end of the device is approach and the projections on one ofsaid members being ribs separated by a channel of which the depth variesperiodically along a, length of the channel.

16. In a machine for the extrusion of. plastic metal, two concentricmembers relatively rotatable about a common axis, projections on theadjacent surfaces of'said members which co-operate to form a screwthread device for impelling plastic metal between sa d Embers theprojections on one of said members comprising ribs separated by achannel of which the depth increases progressively, from a point betweenits ends, as the delivery end of the device is approached and theprojections on the other of said members being ribs separated bya'channel of which the depth varies periodically along a length of thechannel.

17. In a machine for the extrusion of plastic metal, two concentricmembers relatively rotatable about a common axis, projections on theadjacent surfaces of said members which 00- operate to form a screwthread device for impelling plastic metal between said members, theprojections on one of said members comprising ribs separated by achannel of which the depth increases progressively, from a point betweenits ends, as the delivery end of the device is approached and theprojections on one of said members comprising helical ribs separated bya channelof which the depth varies periodically along a length of thechannel and also gradually alters, from a point between its ends, as thedelivery end of the device is approached.

18. In a machine for the extrusion of plastic metal, two concentricmembers relatively rotatable about a common axis, projections on theadjacent surfaces of said members which cooperate to form a screw threaddevice for impelling plastic metal between said members, the projectionsof one of said members comprising ribs separated by a channel of whichthe depth increases progressively, from a point between its ends, as thedelivery end of the device is approached and the projections on theother of said members comprising ribs separated by a channel of whichthe depth varies periodically along a length of the channel andgradually decreases, from a point between the ends of the channels, asthe delivery end of the device is approached.

19. In a machine for the extrusion of plastic metal, two concentricmembers relatively rotatable about a common axis, projections on theadjacent surfaces of said members which cooperate to form a screw threaddevice for impelling plastic metal between said members, the projectionson one of said members comprising helical ribs separated by a channel ofwhich the depth increases progressively from a point between the ends ofthe channel, as the delivery end of the device is. approached, theprojections on the other of said members comprising ribs separated by achannel of which they depth decreases progressively from a point betweenthe ends of the channel, as the delivery end of the device is approachedand the bottom surface of the channels of one of said members being alsoof undulatory form.

20. In a machine for the extrusion of plastic metal, two concentricmembers relatively rotatable about a common axis, projections on theadjacent surfaces of said members which co-operate to form a screwthread device for impelling plastic metal between said members, theprojections on one of said members comprising helical ribs separated bya channel of which the depth increases progressively, from a pointbetween its ends, as the delivery end of the device is approached, andthe projections on the other of said' members comprising ribs separatedby a channel of which the depth decreases progressively, from a pointbetween its ends, as the delivery end of the device is approached and ofwhich the bottom surface is also of undulatory tween its ends, as thedelivery end of the machine proached, and the projections on the otherof said members comprising ribs separated by a channel which is in theform of a helix whose axis is displaced from the said common axis ofrotation and of which the depth varies periodically along the lengththereof.

22. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the said members which cooperate to form a screw threaddevice for impelling plastic metal between said members, the projectionson the first of said members comprising ribs separated by a channel ofwhich the depth increases progressively, from a point between its ends,as the delivery end of the device is approached and the projections onthe second of said members being formed by cutting in the surfacethereof adjacent the first said member at least two helical channels,one of said channels having its axis displaced in one direction from thesaid common axis of rotation which is also the axis of rotation of thesaid surface and the other of said channels having its axis displaced inanother direction from the said common axis of rotation, whereby thedepth of each of the eccentric channels varies periodically along thelength thereof.

23. In a machine for the extrusion of plastic metal, comprising twoconcentric members relatively rotatable about a common axis, ribs on oneof said members separated by a channel of which the depth increasesprogressively, from a point between its ends, as the delivery end of themachine is approached and projections on the other of said membersco-operating with said ribs to form a screw thread device for impellingplastic metal between said members, the provision of a local increase inthe clearance between the said ribs and the said projections.

24. In. a machine for the extrusion of plastic metal, two concentricmembers relatively rotatable about a common axis, projections in theform of ribs on one of said members separated by a channel of which thedepth increases progressively, from a point between its ends as atintervals along the length thereof narrow circumi'erentially extendingslots therein;

25. In a machine for the extrusion of plastic metal, two concentricmembera'means for driv- I ing one of said members in rotation about itsaxis,

means for anchoring the other of said members,

projections in the form of ribs on one-of said.

members separated by a channel of which the depth increasesprogressively, from a point be- -is' approached, projections on theother of said members co-cperatins with the ribs on the firs member toform a screw thread device for impelling plastic metal between saidmembers, and a pre-forming chamber in which the several streams ofplastic metal intermingle before leaving the impelling device, saidchamber being located between the inlet and delivery ends of theimpelling device and formed in the anchored member by providing a localincrease in the clearance between the projections on the anchored memberand those on the driven member.

26. In a machine for the extrusion of plastic metal, two concentricmembers, means for driving one of said members in rotation about itsaxis, means for anchoring the other of said members against rotation,projections in'the form of ribs on one of said members separated by achannel of which the depth increases progressively, from a point betweenits ends, as the delivery end of the machine is approached, projectionson the other of said members co-operating with the ribs on the firstmember to form ascrew thread device for impelling plastic metal betweensaid members, the projections on the anchored member being ribs in eachof which is a transverse groove of considerable width, thereby toprovide between the inlet and delivery ends of the impelling device apre-forming chamber in which the several streams of plastic metalintermingle before leavarated by a channel of which the depth increasesprogressively, from a point between its ends, as the delivery end of themachine is approached, projections on the other of said memberco-opcrating with the ribs on the first member to form a screw threaddevice for impelling plastic metal between said members, the projectionson the anchored member being ribs of helical form in each of which is atransverse groove having a width approximately equal to half the lengthof one complete turn of the rib and having a maximum depth at itsleading edge, thereby to provide near the delivery end of the impellingdevice a pre-forming chamber in which the several streams of plasticmetal intermingle beforeleaving the impelling device.

28. In a machine for the extrusion of plastic metal, two longitudinallyextending concentric members relatively rotatable about a common axis,projections on the said members which cooperate to form a screw threaddevice for impelling plastic metal between said members, the projectionson one of said members comprising helical ribs separated by a channel ofwhich the depth increases progressively from a point between its ends asthe delivery end of the device is approached; an extrusionchamber on thedelivery end of the impelling device; supports extending between theinner of said concentric members and the wall of said chamber, saidsupports having corrugated side walls, the corrugations extending at anacute angle to the general direction of the stream of plastic metalflowing therepast.

29. In a metal extrusion machine as specified in claim 28, thedisposition of the corrugations on 'the' one sidewall of a support at anangle to those on the opposite side wall of that support.

.30. In a machinefor' the extrusion of plastic metal, two longitudinallyextending concentric members; means for driving the outer or saidmembers in rotation about its axis; means for anchoring the other ofsaid members against rotation; helical ribs on said anchored memberseparated by a channel of which the depth in- .creases from a pointbetween its ends, as the delivery end of the machine is approached, saidhelical ribs each having at intervals along its length narrowcircumferentially extending slots and near its delivery end a widetransverse groove having a maximum depth at its leading edge; helicalribs on said driven member separated by a channel of which the depthvaries periodically along a length thereof and also decreases, from a apoint between its ends, as the delivery end of the machine isapproached, said helical ribs on said concentric members co-operating toform a screw thread device for impelling plastic metal between saidmembers,.in which segregation of the plastic metal within the device issubstantially inhibited.

PERCY DUNSHEATH.

